The primary long-term goal of this study is to describe the way in which, on the one hand, mechanical features of the microvessel wall and, on the other, neurogenic and biogenic factors interact in regulating microcirculation in conventional young and aged rats, germfree and hypertensive rats, and gerbils. 1) Venular distensibility gradients required to evoke arteriolar and precapillary sphincters response will be analyzed in mesentery and striated (cremaster) muscle and brain (pial) vasculatures in rat and gerbil. The nature of the venous-arteriolar response will be explored and the consequences of the micro-vascular diametric changes upon flow determined. 2) An assessment will be made of the influence of a selected number of naturally occurring biogenic molecules upon microvascular response to central neutral stimulation in mesentery and skeletal muscle. 3) The extent, if any, of other central neural loci of transmitter(s) release, e.g., nucleous coereleus, on cerebral (pial, cortical) and other tissue microcirculation will be examined.